An example of composition of a conventional 900 MHz-band amplifier for portable telephone that uses a source-grounded MOSFET is shown in FIG.1. The conventional amplifier is composed of MOSFET 1 with a stability factor k of greater than 1 for the device itself in a bandwidth that the amplifier is used, and a matching circuit 2 and a bias supplying circuit 6 and a matching circuit 3 and a bias supplying circuit 7 that are connected to the gate and drain, respectively, of MOSFET 1.
In this composition, when the matching circuits 2, 3 and the gate bias supplying circuit 6 have no loss in high-frequency use, and when the drain bias supplying circuit 7 has no loss to both direct-current and high-frequency, the performance of the amplification device becomes maximum. When the matching circuits 2, 3 or the bias supplying circuits 6, 7 have a loss, the performance of the amplification device reduces with that loss.
Also, when the amplification device in FIG. 1 is MMIC-structured, since all or part of the matching circuits 2, 3 and bias supplying circuits 6, 7 are formed on silicon substrate, the loss increases and it is therefore difficult to enhance its performance.
For the purpose of preventing deterioration in performance due to the loss of matching circuit, it is necessary to use a passive element with a reduced loss as the matching circuit. Further, in order to reduce the loss of a passive element, especially an inductor, it is generally necessary to increase the film thickness of wiring metal or increase the wiring width. To increase the film thickness of wiring metal needs a particular process for making such a thick wiring or takes a very long time for making it. Therefore, the manufacturing cost increases. On the other hand, when the wiring width is increased, especially for a spiral inductor, the size increases outstandingly because the certain number of turns is required to get a necessary inductance. Therefore, the manufacturing cost increases as well. Also, in order to reduce the loss of a capacitor, it is necessary to use a low-resistance material for the electrode. Therefore, the number of process increases.
When a capacitor is formed on a silicon substrate, since the lower electrode of the capacitor and the silicon substrate are in capacitive coupling, a loss due to the resistance of the silicon substrate occurs. In order to prevent this, high-permittivity film or dielectric thin film is needed. Therefore, the manufacturing cost increases as well.
FIG. 2 shows an example of composition of a conventional MMIC amplification device composed of GaAsFET. Also in this device, in order to reduce sufficiently the loss of matching circuit and bias supplying circuit, it is necessary to increase the wiring width of spiral inductors 12, 13 and 15 or to increase the film thickness of wiring metal thereof. In the former solution, the chip cost of MMIC increases, thereby the manufacturing cost increases. The latter solution needs a particular process for making such a thick wiring or takes a very long time for making it. Therefore, the manufacturing cost increases as well.
In the conventional devices, as described above, for the purpose of preventing deterioration in performance due to the loss of matching circuit and bias supplying circuit, it is necessary to increase the film thickness of wiring metal used for matching circuit, to increase the wiring width, or to use high-permittivity film or dielectric thin film for capacitor. Therefore, the manufacturing cost increases.